This invention relates to the regeneration of noble metal hydrogenation catalysts. In one aspect, the invention relates to the regeneration of palladium and platinum catalysts. In another aspect, the invention relates to the regeneration of such catalysts after use in the reduction of organic compounds with nitro moieties to the corresponding amines.
Noble metal catalysts, because of their nature, lose their activity after prolonged use. Since these catalysts are relatively expensive, it is clearly advantageous to regenerate them rather than replace them. The regenerated catalyst should preferably be substantially as active and selective as a freshly prepared catalyst.
It is well-known in the art to use steam or hot gases to volatilize organic liquids and thus remove them from the catalysts. It is also known to subject deactivated catalysts to an oxidizing treatment to reactivate the catalyst. These prior art methods are not very satisfactory as both methods are generally ineffective to restore more than partial catalytic activity.
Processes have also been developed in the art for reactivation of particular noble metal hydrogenation catalysts employed in specific hydrogenation systems. U.S. Pat. No. 3,824,193 discloses that alumina-supported palladium catalysts used in catalyzing the hydrogenation of organic esters can be reactivated by treating the catalyst briefly with an aqueous alkali metal hydroxide solution and then heating the catalyst in air to a temperature between 100.degree. C. and 500.degree. C. Noble metal hydrogenation catalysts used for producing hydrogen peroxide by the anthraquinone process can be regenerated in accordance with U.S. Pat. No. 3,901,822 by contacting the catalyst with a polar organic solvent; then contacting it with an aqueous ammonium hydroxide solution; and finally contacting the catalyst with steam and an oxygen-containing gas at temperatures from about 250.degree. C. to the transition temperature of the catalyst crystal structure. Palladium catalysts employed in the hydrogenation of reducible cyclic organic compounds containing nitrogen impurities are regenerated by contacting the separated catalyst with an agent selected from the group consisting of alkali metal and alkaline earth metal bicarbonates, carbonates, nitrates, chlorides, fluorides and hydroxides in a liquid reaction medium as is taught in U.S. Pat. No. 3,959,382.
The foregoing prior art methods for reactivation of particular hydrogenation catalysts in specific systems are not effective to more than partially regenerate the activity of a highly deactivated catalyst employed in the reduction of organic nitro compounds. It is highly desirable then to provide a method whereby a noble metal catalyst deactivated in the prolonged reduction of organic nitro compounds can be economically regenerated to an activity substantially equivalent to that of a freshly prepared catalyst.